How common were "homemade" armored vehicles in that period? I've seen a few photos of armored cars that were made by attaching sheet metal to the sides of cars and adding a machine gun, but I'm not sure how often this might have been done.

I did that for my Bolshevik forces. I had a Lledo Model T truck that was just taking up space, so I converted it into a field-modified armored car by glueing sheets of plastic over it, adding pop rivets and other details, and adding a turret. It looks very BoB-ish/RCW-ish, but I don't know how realistic it might be.

was the vehicle in the theatre at that time?yes - use it, who can prove one wasn't capturedno - use it, who can prove noone bought/brought one from abroadthe vehicle wasn't even constructed at that time? - use it, a spy stole the blueprints of the prototype and it was built for experiment

of course this rule has to be used with measure, as a king tiger in BoB wouldn't work

one is usually surprised what sorts of stuff showed up in the wierdest places, you never knowor did anyone know that the US experimented with steam tanks in WWI?or that steam airplanes existed?(this was not rhetorical)

Very, very common, especially early in the war. In some theatres they were the dominant type (Estonians versus Freikorps, for example) even quite late.

Mostly armoured cars, often peppered with MGs all around to compensate for no turrets.

The Whites in the south also armoured tracked tractors, to make improvised tanks.

Thank God for that! I've been covering a old diecast cars/buses/truck style vehicles in plasticard to use as 'home-made' armoured cars' for the period and have been quite pleased with the results. Was gonna start using some for SCW and VBCW skirmish games.